194 THE ANCESTRY OF VERTEBRATES 



alimentary tract which meets an invagination of the skin. 

 Perforation, according to GOETTE's well-known represent- 

 ation of a longitudinal section in Bombinator, only occurs 

 when the growth of the tail is well advanced, in Rana 

 temporaria according to BALFOUR somewhat earlier. 



Hertwig (1883, p. 285) also sees the anus in Rana 

 temporaria arise behind and independently from the slit- 

 like blastopore He emphasizes the different behaviour of 

 the germinal layers at the. border of the two structures. 

 At the blastopore border ecito- and entoderm are continuous 

 with the mesoblast and are separated from each father by the 

 latter. At the anus, on the contrary, ecto- and entoderm 

 pass directly into each other, forming a passage which 

 pierces the mesoderm at that place. 



Spencer (1885), on the contrary, comes to the conclusion 

 that the blastopore in Rana temporaria remains open and 

 passes directly into the anus The blastopore is not enclosed 

 by the medullary folds and thus there is no neurenteric 

 canal. The former conclusion is shared by DURHAM (1886) 

 in a short note, but according to the latter, a neurenteric 

 canal is formed independently from the blastopore. In 

 one series, truly, DURHAM finds no blastoporal opening 

 whatever but considers this as a pathological case. Kupffer 

 (1887), dealing with the same object, comes to the conclu- 

 sion that the blastopore remains open as the anus; so, also 

 PERENYI (1888). 



SCHANZ (1887) also operated on Rana temporaria, together 

 with Triton. In Rana he concludes that the medullary 

 folds rather close over the blastopore, that there is indeed 

 a neurenteric canal, though the lumen is not evident, and 

 that the anus arises by perforation at the bottom of a little 

 groove behind it. 



As regards the facts SiDEBOTHAM (1888) quite agrees 

 with him. According to him BALFOUR's description is the 

 right one, he too sees in sections the "diverticulum from 

 the hind end of the mesenteron, dipping down towards 

 a distinct pit in the epiblast below the blastopore and 

 quite separate from it." Eventually perforation ensues 

 Similarly by MORGAN (1890i in Rana halecina and Biifo 

 lentiginosus the anus is seen to arise at the bottom of a 

 little groove in the ectoderm behind the blastopore 



GOETTE (1890), after a renewed investigation on Bom- 

 binator igneus and some other Anurans reaches the con- 



